1. Field of the Invention
The present invention relates to an optical fiber transmission system, a Raman gain slope measuring device and a Raman gain slope measuring method which enable measurement of Raman gain slope of an optical fiber transmission line.
2. Description of the Related Art
(Distributed Raman Amplification)
In the field of communication systems using an optical fiber transmission line, development is under way for commercializing distributed Raman amplification (DRA) techniques. Optical fiber for use in basic transmission networks today employs quartz glass as a base material. Raman amplification is a phenomenon that takes place when a signal light and a pump light having a frequency 13 THz higher than that of the signal light are simultaneously incident on a quartz glass causing part of energy of the pump light to be transferred to the signal light through the induced Raman scattering effect of the quartz glass. As a result, the signal light is subjected to amplification. Gain obtained as a result of Raman amplification will be referred to as a Raman gain hereinafter. An actual Raman gain has a wavelength dependency as shown in FIG. 8, and will be referred to as a Raman gain profile hereinafter.
Distributed Raman amplification is a mode of applying pump light to an optical fiber which transits signal light to obtain the Raman amplification effect with the optical fiber transmission line itself as an amplification medium. Since a propagation loss of a transmission line is compensated for by Raman amplification, an optical fiber transmission system using distributed Raman amplification enables the extension of a distance in which the signal is transmittable.
(Necessity of Measurement of Raman Gain Slope)
A Raman gain (dB), which is generated when pump light of a certain power (W) is applied to an optical fiber as a Raman amplification medium, normalized by the power of the pump light, is referred to as Raman gain slope (dB/W). In the following, a description is given showing the importance of measuring the Raman gain slope in the distributed Raman amplification process.
Raman gain slope varies with an individual fiber. To begin with, optical fibers laid as basic transmission networks have various kinds and Raman gain slope depending on a mode field diameter (core diameter), an amount of GeO2 addition, absorption of water (OH), etc. of the optical fibers. These parameters also vary with the manufacturer, the manufacturing time and the particular lot of the optical fiber.
Another chief factor responsible for Raman gain slope variation is station loss. In a large terminal station or a repeater plant in particular, there exist loses of a few dB from connectors at several sites from the room where a pump light source is placed to the actual transmission line optical fiber. With a transmission system using no distributed Raman amplification, station loss can be taken into consideration together with a section loss. In distributed Raman amplification, however, a loss caused before the pump light reaches the transmission line optical fiber is special and therefore needs another specification.
Thus, when distributed Raman amplification is conducted on an existing transmission line whose parameters affecting gain vary widely, it is difficult to predict in advance the pump power required for obtaining a desired Raman gain. Adjustment at the site is therefore needed which costs labor and time.
Elimination of the need for adjustment could be realized if conditions of a site such as properties of a transmission line optical fiber and loss characteristics of a terminal station or a repeater plant can be measured as Raman gain slope. This enables pump power required for obtaining a certain gain to be measured with high precision, thereby enabling a Raman gain to be controlled dynamically.
Raman gain exhibits the configuration as shown in FIG. 8 with respect to a wavelength of signal light. Raman gain slope is a function of a frequency difference between the pump light and the test light. In a typical transmission line made of quartz glass optical fibers, Raman gain slope has a similar figure in the direction of a gain axis. Since a frequency difference at which a gain has a peak is about 13 THz, gain slope of a certain transmission line will be denoted as a signal wavelength 13 THz larger than the pump wavelength unless indicated otherwise in the following.
(Conventional Raman Gain slope Measuring Method)
As described above, while actual measurement of Raman gain slope is crucial, there exists no simple actual method for making such measurements at site.
When work at opposite ends of a transmission line is possible as in a laboratory, measurement of Raman gain slope can be realized by such arrangement as shown in FIG. 6. Provided at one end of a transmission line optical fiber 100 is a test light source 60 having a signal wavelength band whose Raman gain slope is to be measured and provided at the other end is a WDM (Wavelength Division Multiplexing) coupler 110 for multiplexing and demultiplexing a pump wavelength band and a signal wavelength band. A pump light source 120 is connected to a pump wavelength band port of the WDM coupler 110 and a light receiver 130 for measuring power of the signal light is connected to a signal wavelength band port. Examples of the light receiver 130 are such measuring apparatuses as an optical spectrum analyzer and an optical power meter and also a simple light receiving element such as a photo-diode of suitable performance.
With the test light applied to a transmission line and the pump light stopped, one first measures the power of the test light (P1[dBm]) detected at the light receiver 130. Next, with the pump light source 120 output, measure power (P2[dBm]) of the test light detected at the light receiver 130. A difference between the powers P2 and P1 of the test light is a Raman gain (dB) that the test light obtains. One may normalize the gain by the power of the pump light to obtain the Raman gain slope (dB/W). Since this method allows measurement with a simple light receiving element, it can be realized at relatively low cost. It, however, has a problem in that the method is not easy to perform because the opposite ends of a transmission line are apart from each other in general.
(Conventional Raman Gain slope Measuring Method Realized by Work Only at One End of Transmission Line)
Work performed only at one end of the transmission line, such as illustrated in FIG. 7, also enables measurement of Raman gain slope. In this method, a Raman gain is measured by using an OTDR (optical time domain reflectometer) 160. The OTDR 160 is a means for detecting a failure point of an optical fiber by time-resolution measurement of an amount of relay scattered light of an optical pulse which enters the optical fiber and is scattered and returned. Shown in FIG. 7 is a structure in which test light returned scattering is detected by the OTDR 160 through a wavelength filter 140 and an optical attenuator 150. Since with a Raman gain, relay scattered light is increased by as much as the Raman gain, the Raman gain can be measured.
The embodiment of FIG. 7, however, requires a special and expensive OTDR device to be provided at a DRA installation site. The method also requires a temporary change of the transmission line wiring for connecting the OTDR device, which invites problems in costs and workability.
Measuring the Raman gain slope using the conventional method shown in FIG. 6 requires disposition of measuring apparatuses, light sources and workers at opposite ends of a transmission line. On the other hand, although such a conventional method as shown in FIG. 7 enables measurement of Raman gain slope even when work is possible only at one end of a transmission line, it requires an expensive OTDR device to be provided at the site and also transmission line wiring to be changed for connecting the OTDR device, leaving problems to be solved in costs and workability.